Green roofing and green walls can provide many benefits to building managers and occupants as well as to local ecosystems. In natural ecosystems, vegetation and soil often affect the flux of moisture and of heat. When coupled with man-made structures, the services that vegetation provide can be harnessed for the mutual benefit of humans and the environment. Benefits include a reduction in storm surge, a potential lessening of the urban heat island effect, and lowered energy costs for the underlying building.
A living, vegetative layer atop a building behaves very differently than a standard, impervious roof. Storm surge normally associated with impervious surfaces is avoided due to the absorbent capabilities of soil which eases the strain on municipal stormwater systems meaning that smaller, less expensive systems can be installed. Additionally, evaporation and transpiration remove both excess moisture and heat from the roof surface.
However, in green roof applications, soil depth and, subsequently, plant type are limited due to the load-bearing capacity of roofs. Thus, drought tolerant plant species in thin-layer soils are often used. Soils with a high organic matter content help to retain enough moisture and support vegetation, but in warmer climates, organic matter decomposes quickly and soils lose water holding capacity and fertility over time.
Living layers on the exterior walls of buildings may provide additional benefits in terms of heat and humidity exchange and as insulation, and may help to visually integrate the building into the surrounding natural environment. Freestanding outdoor living walls may also be constructed for privacy screening, for shade, or for wind or erosion control.
Inside building structures, living layers on interior walls can provide similar benefits to the interior environment. A living wall may help maintain comfortable levels of humidity and oxygen through plant respiration, may insulate room walls, and may reduce the level of indoor pollutants through biofiltration.
These living wall structures are typically limited in soil depth and plant type, due to the load-bearing capacity of the walls and the required underlying supports. The growth medium used in living walls may also be further limited by the requirement for a vertical structure to keep the growth medium in place.
Soils have typically been used as growth media for green roofs and green walls, but present several issues. Soils can be categorized as mineral soils or organic soils as defined by the USDA in Keys to Soil Taxonomy 11th edition, USDA Natural Resources Conservation Service. Mineral soils typically contain less than 20% by weight organic material. Mineral soils are further distinguished by their clay, sand, and silt content as described in Brady, N.C., 1984, The Nature and Properties of Soils, 9th ed., MacMillan Publishing Company, New York, N.Y. Clay, sand, and silt are formed for the most part by physical or chemical weathering processes from rock, and retain the inorganic crystalline structure of the parent rocks from which they were formed. A large proportion of the mineral content in most soils is composed of silicates derived from the weathering of quartz.
Mineral soils tend to be poor substrates for plant growth. The minerals are typically hydrophobic and retain little water, and water flowing through the soil tends to dissolve and wash away inorganic plant nutrients. Being granular structures, mineral soils also tend to compress, leaving little room for root growth. Mineral soil can be improved for plant growth by adding organic material or biomass to the soil. The presence of hydrophilic polysaccharides in the organic matter increases the water retention and therefore the soluble mineral nutrient retention of the soil, while the cellular structure of the organic material reduces the tendency to compress, giving the roots space to grow. The organic matter also provides support for insect, annelid, bacterial, and microbial growth, slowly releasing the nutrients tied up in the biomass to the surrounding soil, adding to the organic content of the soil by the accumulation of biomass, and further decompressing the mineral soil by digging and tunneling or by growing against the mineral grains. The presence of microorganisms and other growth on the decomposing organic matter also helps to bind the mineral grains together, reducing erosion.
Organic soils are in general less dense, more nutritious for plant life, and more water retentive than mineral soils. Water retention is a measure both of the mass of water that can be retained by a soil and of the speed with which it evaporates from the soil. Organic soils also tend to retain more of the mineral nutrients required for healthy plant growth, both as a function of their water retention and in the microbes and bacteria responsible for the decomposition of the organic material.
The bulk dry density of the soil is a major factor limiting the use of soils in green wall or green roof applications. Soil is a three phase system consisting of the solid particles described above, liquid water held within the pore spaces by capillary forces and containing dissolved minerals, and gases in the empty spaces between particles not occupied by liquid. Soil density can be measured in at least three ways. The particle density of the soil is the density of the particles exclusive of the water and gas phases and can be defined as the mass of the solid particles divided by the volume occupied by the solid particles. The bulk dry density can be defined as the mass of the solid particles divided by the total volume occupied by the solid particles, the water, and the gas. The total density or wet density can be defined as the combined mass of solid particles and liquid divided by the total volume occupied by the solid particles, the liquid, and the gas. The wet density is highly variable due to the fluctuation of the moisture content and difficulty in measuring the moisture content of the soil. A more thorough description of the calculation of properties describing soils can be found in Yu et al, Data Collection Handbook To Support Modeling Impacts Of Radioactive Material In Soil, Environmental Assessment and Information Sciences Division, Argonne National Laboratory, Argonne, Ill. (1993), incorporated herein by reference in its entirety.
The dry bulk densities of most soils range from about 1100 kg/m3 to about 1600 kg/m3. This density of soil limits the thickness of the layer that can be supported by the underlying construction of the architectural structure, which further limits the choice of plants. Not only is the length of the plant root restricted due to the shallower soil, but the evaporation of water from the soil is a function of the surface area, and the thinner the soil layer, the more rapid the evaporation of water compared to the volume of the soil. These thin soil layers typically necessitate small, drought tolerant plants with shallow root systems. Because erosion is also mainly a factor of the surface area of the soil and because the plants are already limited by the thickness of the soil layer, thin layers of soil must usually be replaced or supplemented more frequently than thicker layers.
One growth medium used for both green roofs and living walls includes a combination of peat moss and soil, intended both to reduce the density of the soil and thus the weight of the soil layer, and to increase the organic content of the soil and thus the water retention and nutritive value of the soil. Peat moss is a low density organic material found predominately in natural deposits in shallow wetland areas. Depending on the type of the peat moss, dry bulk density may range between about 160 kg/m3 to about 600 kg/m3, while a more compressed form called humus may have a density up about 1000 kg/m3. However, peat moss is generally flammable and often renders the entire structure unacceptable or unsafe. Peat moss has an autoignition temperature of 260° C., which means that even in the absence of a spark or flame, the material heated to that temperature could spontaneously combust. Additionally, peat moss and other types of fiber or matting used for the same purpose typically decompose rapidly and must be replaced frequently. This decomposition in itself produces heat, which has in some cases been blamed for the spontaneous fires in stored peat moss. Although the mineral portions of the soil are not generally flammable, addition of peat moss, fiber, matting, and other forms of biomass such as mulch and shredded plant matter typically result in undesirably flammable material.
Fires in peat moss may be especially dangerous due to the burning characteristics of the materials. Peat moss fires tend to smolder for long periods of time, frequently erupting in hot spots if the surface is broken through, and produce large amounts of carbon monoxide. In many cases, peat fires are simply allowed to burn until all of the peat is consumed due to the dangers involved to firefighters, and fires in naturally occurring peat deposits have been allowed to burn for years.
Additionally, soil, especially on non-horizontal surfaces such as sloped roofs or vertical walls, exhibits erosion, during which particles of dirt move down the surface of the roof or spill out of the wall onto lower surfaces under the action of gravity, wind, or water flow. This necessitates frequent replacement or supplementation of the growth media. In some applications, fiber matting, cloth bags, and wooden supports have been used to control erosion, but all of these solutions increase the risk of fire and although erosion may be slowed it is still an important source of loss and necessary replacement of the growth medium.
Soil in itself is not an especially desirable material for construction of roof and wall structures. The unattached granular structure results in erosion and loss of material, which necessitates constant renewal and replacement, and especially in interior applications results in a constant need for cleaning surrounding areas. Handling the soil is a messy job, both in the initial construction and in the necessary renewals, resulting in spills, tracking, and loss. Maintaining the cleanliness of interior and exterior spaces near a soil roof or wall is a time consuming operation.
Soil is also a poor insulator. Depending on the source, the R-value for soil is variously reported as being between R-0.25 and R-1.0. R-value is a measure of the thermal resistance of a material, reported in the US in the units h·° F.·ft2/(BTU·in). The R-value is the inverse of the thermal conductivity, which measures the rate of heat transfer through a building element over a given area under standardized conditions. The higher the R-value, the slower heat passes through the building element. For comparison, the R-value for fiberglass batting typically ranges between about R-3 and about R-5, which implies that a layer of soil would need to be at least from 3 to 5 times to about 12 to 40 times as thick as a layer of fiberglass batting to provide the same insulation. R-values of many materials have been measured, are frequently used in advertising of insulations, and can be found in multiple easily available sources in the building trades.
The R-value of an insulative material is not the only factor that affects the transfer of heat through the material. The presence of a vegetative layer, water in the soil, and a large thermal mass presented by the thickness of soil required to maintain a vegetative layer do modify the low insulative value of the soil somewhat. The transpiration of the vegetative layer and the evaporation or condensation of water in the soil layer also help to modify the transfer of heat. The thermal modifications of the vegetative layer and the presence of water are some of the drivers that have led to the development of soil-based green roofs in spite of the problems inherent in using soil as a substrate.
What is desired, therefore, is a growth media for green roofs and green walls that would show reduced erosion, reduced flammability, and reduced thermal conductivity compared to either mineral soil or organic soil media. It is also desired that any replacement growth media would retain sufficient water and nutrients to support a healthy, long-lasting vegetative layer.
For some limited applications, it has been proposed to use foam substrates as media for plant propagation which contain no soil. Plant propagation is sensitive to erosion, because watering recently planted seeds or cuttings can wash the mineral particles away from roots that have not yet established a firm support, or in some cases can physically wash the seeds away from the area where they are planted. This can result in disruption of the establishment of a root system and the death of the seed or cutting. Plant propagation is also sensitive to water retention; if the soil dries out too quickly, the new roots will wither before they are established, and if it stays too wet, they will rot. Dietrich, et al (U.S. Pat. No. 3,838,075) describe a hydrophilic betaine foam for plant propagation which has a density ranging between about 6 kg/m3 and about 50 kg/m3. In the embodiment suggested by Dietrich, the foam is cast into small elements, charged with seeds, and compressed with a solution of adhesive to hold the elements in a compressed form until planted. The elements can then be planted out in soil and serve as a safe spot in which the seed can germinate, with erosion and water retention controlled by the foam rather than the surrounding soil. Once the seed has germinated, the roots then grow into the soil giving the plant the necessary structure to continue growth. The elements described by Dietrich can also be preloaded with nutrients and worked into soil to serve as a soil amendment, adding both plant nutrients and water retention to the surrounding soil.
A similar hydrophilic foam has been disclosed by Wood et al (U.S. Pat. No. 3,889,417), in the form of a foam sheet used as a carrier for seeds, herbicides, pesticides, and the like. The sheet is typically either spread on or dug into the existing soil and can serve as a safe place for plant germination, after which the plant establishes itself on the underlying soil. However, plants quickly outgrow these small elements, and neither of these provides sufficient support for plants to grow and thrive for a useful span of time in the absence of underlying soil.
To address the flammability of polyurethane foams, flame retardant polyurethanes have recently been developed incorporating large quantities of alumina trihydrate in the hydrophilic polyurethane foams, for example Marans et al (U.S. Pat. No. 4,165,411) and Murch et al (U.S. Pat. No. 4,365,025). Unfortunately, high levels of aluminum have been shown to be highly toxic to plant root plasma membranes and to inhibit cell division, cell extension, and transport. See for example Mossor-Pietraszewska, “Effect of aluminium on plant growth and metabolism”, Acta Biochimica Polonica 48(3), 673-686.
What is desired, therefore, is a growth media that can serve as a support for a healthy vegetative layer in the absence of underlying or surrounding soil, and that has reduced flammability.